1. Field of the Invention
The present invention relates to circuits, and, in particular, to adaptive de-emphasis setting for backplanes and the like.
2. Description of the Related Art
Many communication systems employ transmission of data from a transmitter to a receiver through a transmission media. In serial communication applications, components such as integrated circuits (ICs) are typically mounted on a circuit board and are electrically coupled to each other by a plurality of circuit board conductors, commonly known as traces, which may be on a printed circuit board or on a backplane. Traces provide a transmission media for signals, such as power, ground references, and input/output signals to and from each component. Normally, high-frequency signals between components via the traces are adversely affected by parasitic resistance, inductance, and capacitance inherent in such interconnections. Further, impedance mismatches between a transmitter and the various signal transmission media (traces and other circuit board components) of the signal transmission path, as well as between a receiver and the various signal transmission media of the signal transmission path, may produce signal reflections at the transmitter end and/or the receiver end of the signal transmission path. Such signal reflections may propagate along the transmission path and may potentially degrade system performance. As a result, it is highly desirable to closely match the output impedance of the transmitter circuit to the various components including the signal transmission path, and the input impedance of the receiver.
Generally, there is a trade-off between the length of the transmission media and the bit error rate (BER). This trade-off occurs because the transmission media causes frequency distortion that contributes to inter-symbol interference. Consequently, some applications employ a de-emphasis circuit to condition signals prior to being applied to the transmission media. The de-emphasis circuit is a form of transmitter equalization circuit that pre-distorts an input signal to compensate for at least some of the frequency distortion in the data that is caused by the transmission line in the data link. Compensation for the frequency distortion at the output of the transmission line flattens the amplitude response of the output signal, and thereby improves the bit error rate (BER).
FIG. 1 shows a block diagram of a de-emphasis circuit 100 for backplane and cable applications of the prior art. Driver 102 is employed to receive an input data signal, provide gain (e.g., current gain) to the signal, and provide the data signal to transmission (TX) media 104 for transmission to receiver (RX) 106. As shown in FIG. 1, driver 102 provides current gain through the combination of current sources 108 and 110. If 100% of the current flows through current source 108, then adjustment of the percentage of current flowing through current source 110 adjusts the current provided to driver 102 that drives the output data signal. Latch 112 (shown as a D flip-flop) and XOR gate 114 provide control of the current source 110 that applies the de-emphasis, changing the de-emphasis setting depending on whether there is a transition in the input data bit sequence.
FIG. 2 shows a graph of exemplary de-emphasis selected to compensate for dispersion of the transmission stage media where multiple levels of de-emphasis might be selected based on number of bits of a given type received. As shown, the appropriate de-emphasis changes depending on whether a transition in the data bits is detected, since a transition (rise or fall) generally requires greater gain from the driver. Therefore, for the first bit after a transition, greater gain is required, with each additional n'th bit of the same value requiring less gain.
FIG. 3 shows an exemplary circuit schematic of the prior art allowing for adjustable de-emphasis using user-programmed de-emphasis through adjustable current sources. Similar to the circuit of block diagram of FIG. 1, FIG. 3 shows differential driver 302 coupled to combination of current source 308 and current sources 310a, 310b, and 310c. Differential latch 312 (shown as a D flip-flop) and differential XOR gate 314 provide control of the current sources 310a, 310b, and 310c based on the presence or absence in transitions of the input data to driver 302. Current sources 310a, 310b, and 310c operate in a similar manner to that of current source 110 of FIG. 1, except that, with three current sources, each of current sources 310a, 310b, and 310c might be separately enabled or disabled through control signals (Vcontrol1, Vcontrol2, and Vcontrol3) to provide multiple levels of de-emphasis (shown as User Programmed De-emphasis 0dB, 1.8dB, 3.5dB, and 6.2dB). Such exemplary circuit of FIG. 3 allows appropriate de-emphasis setting for different values of gain depending on whether the first bit after transition was detected or if subsequent bits were detected.